Currently, for non-aqueous electrolyte secondary batteries, many researches are conducted for lithium secondary batteries having high-voltage and high energy density. For the positive electrode active material in lithium secondary batteries, a lithium-containing transition metal oxide such as LiCoO2, for example, is generally used, and for the negative electrode active material in lithium secondary batteries, carbon materials for example are generally used. Further, for the non-aqueous electrolyte used in non-aqueous electrolyte secondary batteries, a non-aqueous electrolyte obtained by dissolving a solute in a non-aqueous solvent is generally used. For the non-aqueous solvent, cyclic carbonate, acyclic carbonate, and cyclic carboxylate are used, for example, and for the solute, lithium hexafluorophosphate (LiPF6) and lithium tetrafluoroborate (LiBF4) are used, for example.
To improve battery characteristics, there have been attempts to mix various additives into the positive electrode active material, negative electrode active material, and electrolyte, respectively. For example, aiming for an improvement of charge and discharge cycle characteristics, Japanese Laid-Open Patent Publication No. 2003-151621 and Japanese Laid-Open Patent Publication No. 2003-031259 proposed to add vinylene carbonate (VC) or vinyl ethylene carbonate (VEC) into the electrolyte so that VC or VEC decomposes on the negative electrode to form a protective coating, to suppress a side reaction of the electrolyte and the negative electrode active material.
Aiming to improve cycle reliability and storage stability of lithium secondary batteries, Japanese Laid-Open Patent Publication No. Hei 09-035746 proposed to include an unsaturated cyclic hydrocarbon compound such as 1,5-cyclooctadiene in the electrolyte, so that 1,5-cyclooctadiene and the like and lithium ion in the form of solvates can be co-intercalated into layers of negative electrode carbon.
Further, aiming to suppress a decrease in thermal stability of positive electrode active material and to improve safety under over-charged state, Japanese Laid-Open Patent Publication No. 2001-015158 proposed to include 2,3-dimethyl-1,3-butadiene in the electrolyte so that 2,3-dimethyl-1,3-butadiene can be polymerized by electrolysis on the positive electrode while in over-charged state.
However, when a material system which undergoes a great volume change while charging and discharging, such as a material composed mainly of Si, Sn, or the like as an element for example, is used for the negative electrode active material, despite using any of the above-proposed prior art techniques, the coating formed on the surface of the negative electrode active material is damaged or broken. Thus, there was a problem in that the effect intended in the above patent publication documents cannot be obtained sufficiently.
The present invention is made to solve the conventional problems such as the above, and aims to improve various characteristics of non-aqueous electrolyte secondary batteries by forming a coating which durably exists on the surface of the negative electrode active material without being damaged or broken regardless of the degree of the volume change of the negative electrode active material.